A little late, I know...but I have had some luck with TunerPro RT in talking to my 94 ecm. I believe there's a program called winflash on the site that sells obd I and II cables. That program, in theory, can extract your bin from the ecm. You then change the speedo ratio using TunerPro, save changes (to a new bin file, imho) and then flash the corrected bin back into your ecm.
THIS IS ONLY THEORY!!!!
All I've been able to accomplish so far is learn that I still had freon in my AC!!! Lol
But there's potential there. 94 and 95 had flash based proms and can be reprogrammed.
I wanna find me a 98 rear axle with the Auburn and discs for the project. But getting a running engine is first on the list.... Work has had me busy lately!!!